diff --git a/src/app/Models/Document.php b/src/app/Models/Document.php
index ae17803..5b0fb1b 100644
--- a/src/app/Models/Document.php
+++ b/src/app/Models/Document.php
@@ -3,65 +3,25 @@
 namespace App\Models;
 
 use App\Helpers\SlugHelper;
+use Illuminate\Database\Eloquent\Builder;
 use Illuminate\Database\Eloquent\Factories\HasFactory;
 use Illuminate\Database\Eloquent\Model;
-use Illuminate\Database\Eloquent\SoftDeletes;
 use Illuminate\Database\Eloquent\Relations\BelongsTo;
 use Illuminate\Database\Eloquent\Relations\HasMany;
 use Illuminate\Database\Eloquent\Relations\HasManyThrough;
-use Illuminate\Database\Eloquent\Builder;
+use Illuminate\Database\Eloquent\Relations\HasOne;
+use Illuminate\Database\Eloquent\SoftDeletes;
+use Illuminate\Support\Facades\App;
 use Illuminate\Support\Facades\Storage;
-use Illuminate\Support\Str;
-use League\CommonMark\CommonMarkConverter;
-use League\CommonMark\Extension\CommonMark\Node\Inline\Link;
-use League\CommonMark\Extension\GithubFlavoredMarkdownExtension;
 
 class Document extends Model
 {
     use HasFactory, SoftDeletes;
 
-    /**
-     * Get the route key for the model.
-     *
-     * @return string
-     */
-    public function getRouteKeyName(): string
-    {
-        return 'slug';
-    }
-
-    /**
-     * Retrieve the model for a bound value.
-     * Supports both slug and ID for backwards compatibility.
-     *
-     * @param  mixed  $value
-     * @param  string|null  $field
-     * @return \Illuminate\Database\Eloquent\Model|null
-     */
-    public function resolveRouteBinding($value, $field = null)
-    {
-        // First try to find by slug
-        $document = $this->where('slug', $value)->first();
-        
-        // If not found by slug, try by ID (for backwards compatibility)
-        if (!$document && is_numeric($value)) {
-            $document = $this->where('id', $value)->first();
-        }
-        
-        return $document;
-    }
-
-    /**
-     * The attributes that are mass assignable.
-     *
-     * @var list<string>
-     */
     protected $fillable = [
         'path',
-        'title',
         'slug',
-        'content',
-        'rendered_html',
+        'default_locale',
         'frontmatter',
         'file_size',
         'file_hash',
@@ -70,11 +30,6 @@ public function resolveRouteBinding($value, $field = null)
         'updated_by',
     ];
 
-    /**
-     * Get the attributes that should be cast.
-     *
-     * @return array<string, string>
-     */
     protected function casts(): array
     {
         return [
@@ -83,192 +38,64 @@ protected function casts(): array
         ];
     }
 
-    /**
-     * Frontmatterをパース（互換性のため残す）
-     *
-     * @param string $content
-     * @return array{frontmatter: array, content: string}
-     */
-    protected static function parseFrontmatter(string $content): array
+    public function getRouteKeyName(): string
     {
-        $frontmatter = [];
-        $bodyContent = $content;
+        return 'slug';
+    }
 
-        // Frontmatterの検出（--- で囲まれた部分）
-        if (preg_match('/^---\s*\n(.*?)\n---\s*\n(.*)/s', $content, $matches)) {
-            $frontmatterText = $matches[1];
-            $bodyContent = $matches[2];
+    public function resolveRouteBinding($value, $field = null)
+    {
+        $document = $this->where('slug', $value)->first();
 
-            // 簡易的なYAMLパース（key: value形式のみ）
-            $lines = explode("\n", $frontmatterText);
-            foreach ($lines as $line) {
-                if (preg_match('/^([^:]+):\s*(.*)$/', $line, $lineMatches)) {
-                    $frontmatter[trim($lineMatches[1])] = trim($lineMatches[2]);
-                }
-            }
+        if (!$document && is_numeric($value)) {
+            $document = $this->where('id', $value)->first();
         }
 
-        return [
-            'frontmatter' => $frontmatter,
-            'content' => trim($bodyContent),
-        ];
+        return $document;
     }
 
     /**
-     * Markdownをレンダリング
-     *
-     * @param string $markdown
-     * @return string
+     * Backward-compatible static delegate so existing callers and tests
+     * (e.g. MediaEmbedExtensionTest) keep working.
      */
     public static function renderMarkdown(string $markdown): string
     {
-        $converter = new CommonMarkConverter([
-            'html_input' => 'strip',
-            'allow_unsafe_links' => false,
-        ]);
-
-        $converter->getEnvironment()->addExtension(new GithubFlavoredMarkdownExtension());
-        $converter->getEnvironment()->addExtension(new \App\Markdown\MediaEmbedExtension());
-
-        return $converter->convert($markdown)->getContent();
+        return DocumentTranslation::renderMarkdown($markdown);
     }
 
-    /**
-     * [[wiki-link]]を抽出してリンクテーブルに同期
-     *
-     * @return void
-     */
-    public function syncLinks(): void
+    // ----- Relations -----
+
+    public function translations(): HasMany
     {
-        // 既存のリンクを削除
-        $this->outgoingLinks()->delete();
-
-        // [[wiki-link]]を抽出
-        preg_match_all('/\[\[([^\]]+)\]\]/', $this->content, $matches);
-
-        if (empty($matches[1])) {
-            return;
-        }
-
-        $position = 0;
-        foreach ($matches[1] as $linkTitle) {
-            $linkTitle = trim($linkTitle);
-
-            // リンク先のドキュメントを検索
-            $targetDocument = static::where('title', $linkTitle)
-                ->orWhere('slug', SlugHelper::generate($linkTitle))
-                ->first();
-
-            DocumentLink::create([
-                'source_document_id' => $this->id,
-                'target_document_id' => $targetDocument?->id,
-                'target_title' => $linkTitle,
-                'position' => $position++,
-            ]);
-        }
+        return $this->hasMany(DocumentTranslation::class);
     }
 
-    /**
-     * [[wiki-link]]をHTMLリンクに変換
-     *
-     * @return string
-     */
-    public function processLinks(): string
+    public function defaultTranslation(): HasOne
     {
-        return preg_replace_callback(
-            '/\[\[([^\]]+)\]\]/',
-            function ($matches) {
-                $linkTitle = trim($matches[1]);
-                $slug = SlugHelper::generate($linkTitle);
-
-                // リンク先のドキュメントを検索
-                $targetDocument = static::where('title', $linkTitle)
-                    ->orWhere('slug', $slug)
-                    ->first();
-
-                if ($targetDocument) {
-                    return '<a href="' . route('documents.show', $targetDocument->slug) . '" class="wiki-link">' . e($linkTitle) . '</a>';
-                } else {
-                    return '<a href="' . route('documents.create') . '?title=' . urlencode($linkTitle) . '" class="wiki-link wiki-link-new">' . e($linkTitle) . '</a>';
-                }
-            },
-            $this->rendered_html
-        );
+        return $this->hasOne(DocumentTranslation::class)
+            ->whereColumn('locale', 'documents.default_locale');
     }
 
-    /**
-     * 全文検索スコープ
-     *
-     * @param Builder $query
-     * @param string $searchTerm
-     * @return Builder
-     */
-    public function scopeSearch(Builder $query, string $searchTerm): Builder
-    {
-        return $query->whereRaw(
-            'MATCH(title, content) AGAINST(? IN BOOLEAN MODE)',
-            [$searchTerm]
-        );
-    }
-
-    /**
-     * ディレクトリ内検索スコープ
-     *
-     * @param Builder $query
-     * @param string $directory
-     * @return Builder
-     */
-    public function scopeInDirectory(Builder $query, string $directory): Builder
-    {
-        $directory = rtrim($directory, '/') . '/';
-        return $query->where('path', 'like', $directory . '%');
-    }
-
-    /**
-     * 作成者リレーション
-     *
-     * @return BelongsTo
-     */
     public function creator(): BelongsTo
     {
         return $this->belongsTo(User::class, 'created_by');
     }
 
-    /**
-     * 更新者リレーション
-     *
-     * @return BelongsTo
-     */
     public function updater(): BelongsTo
     {
         return $this->belongsTo(User::class, 'updated_by');
     }
 
-    /**
-     * 発リンク（このドキュメントから他へのリンク）
-     *
-     * @return HasMany
-     */
     public function outgoingLinks(): HasMany
     {
         return $this->hasMany(DocumentLink::class, 'source_document_id');
     }
 
-    /**
-     * 被リンク（他のドキュメントからこのドキュメントへのリンク）
-     *
-     * @return HasMany
-     */
     public function incomingLinks(): HasMany
     {
         return $this->hasMany(DocumentLink::class, 'target_document_id');
     }
 
-    /**
-     * このドキュメントを最近閲覧したユーザー
-     *
-     * @return HasManyThrough
-     */
     public function recentByUsers(): HasManyThrough
     {
         return $this->hasManyThrough(
@@ -281,57 +108,78 @@ public function recentByUsers(): HasManyThrough
         );
     }
 
-    /**
-     * Translations of this document, one per locale.
-     * (Other relation/accessor refactor lands in Task 4.)
-     */
-    public function translations(): \Illuminate\Database\Eloquent\Relations\HasMany
+    // ----- Translation helpers -----
+
+    public function translationFor(string $locale, bool $fallback = true): ?DocumentTranslation
     {
-        return $this->hasMany(DocumentTranslation::class);
+        $translation = $this->translations->firstWhere('locale', $locale);
+
+        if (!$translation && $fallback) {
+            $translation = $this->translations->firstWhere('locale', $this->default_locale);
+        }
+
+        return $translation;
+    }
+
+    public function isFallback(string $requestedLocale): bool
+    {
+        return $this->translations->firstWhere('locale', $requestedLocale) === null;
     }
 
     /**
-     * ディレクトリパスを取得
-     *
-     * @return string
+     * @return array<int, string>
      */
+    public function availableLocales(): array
+    {
+        return $this->translations->pluck('locale')->all();
+    }
+
+    // ----- Accessors (current-locale → fallback) -----
+
+    public function getTitleAttribute(): string
+    {
+        return $this->translationFor(App::getLocale())?->title ?? '';
+    }
+
+    public function getContentAttribute(): string
+    {
+        return $this->translationFor(App::getLocale())?->content ?? '';
+    }
+
+    public function getRenderedHtmlAttribute(): ?string
+    {
+        return $this->translationFor(App::getLocale())?->rendered_html;
+    }
+
+    // ----- Path helpers -----
+
     public function getDirectoryAttribute(): string
     {
         return dirname($this->path);
     }
 
-    /**
-     * ファイル名を取得
-     *
-     * @return string
-     */
     public function getFilenameAttribute(): string
     {
         return basename($this->path);
     }
 
-    /**
-     * 絶対パスを取得
-     *
-     * @return string
-     */
     public function getAbsolutePathAttribute(): string
     {
         return Storage::disk('markdown')->path($this->path);
     }
 
-    /**
-     * タイトルセット時にslugも自動生成
-     *
-     * @param string $value
-     * @return void
-     */
-    public function setTitleAttribute(string $value): void
-    {
-        $this->attributes['title'] = $value;
+    // ----- Search scope (delegates to translations) -----
 
-        if (empty($this->attributes['slug'])) {
-            $this->attributes['slug'] = SlugHelper::generate($value);
-        }
+    public function scopeSearch(Builder $query, string $term): Builder
+    {
+        return $query->whereHas('translations', function (Builder $q) use ($term) {
+            DocumentTranslation::scopeSearch($q, $term);
+        });
+    }
+
+    public function scopeInDirectory(Builder $query, string $directory): Builder
+    {
+        $directory = rtrim($directory, '/') . '/';
+        return $query->where('path', 'like', $directory . '%');
     }
 }
diff --git a/src/tests/Unit/Models/DocumentTest.php b/src/tests/Unit/Models/DocumentTest.php
new file mode 100644
index 0000000..ba36287
--- /dev/null
+++ b/src/tests/Unit/Models/DocumentTest.php
@@ -0,0 +1,79 @@
+<?php
+
+namespace Tests\Unit\Models;
+
+use App\Models\Document;
+use App\Models\DocumentTranslation;
+use Illuminate\Foundation\Testing\RefreshDatabase;
+use Illuminate\Support\Facades\App;
+use Tests\TestCase;
+
+class DocumentTest extends TestCase
+{
+    use RefreshDatabase;
+
+    public function test_title_accessor_returns_current_locale_translation(): void
+    {
+        $doc = Document::factory()->create(['default_locale' => 'en']);
+        $doc->translations()->where('locale', 'en')->update(['title' => 'Hello']);
+        DocumentTranslation::factory()->create([
+            'document_id' => $doc->id,
+            'locale' => 'ja',
+            'title' => 'こんにちは',
+        ]);
+
+        App::setLocale('ja');
+        $this->assertSame('こんにちは', $doc->fresh()->title);
+
+        App::setLocale('en');
+        $this->assertSame('Hello', $doc->fresh()->title);
+    }
+
+    public function test_title_accessor_falls_back_to_default_locale(): void
+    {
+        $doc = Document::factory()->create(['default_locale' => 'en']);
+        $doc->translations()->where('locale', 'en')->update(['title' => 'Hello']);
+
+        App::setLocale('ja');
+        $this->assertSame('Hello', $doc->fresh()->title);
+    }
+
+    public function test_content_and_rendered_html_accessors_fall_back(): void
+    {
+        $doc = Document::factory()->create(['default_locale' => 'en']);
+        $doc->translations()->where('locale', 'en')->update([
+            'content' => 'English body',
+            'rendered_html' => '<p>English body</p>',
+        ]);
+
+        App::setLocale('ja');
+        $fresh = $doc->fresh();
+        $this->assertSame('English body', $fresh->content);
+        $this->assertSame('<p>English body</p>', $fresh->rendered_html);
+    }
+
+    public function test_is_fallback_returns_true_when_locale_missing(): void
+    {
+        $doc = Document::factory()->create(['default_locale' => 'en']);
+        $this->assertTrue($doc->isFallback('ja'));
+        $this->assertFalse($doc->isFallback('en'));
+    }
+
+    public function test_translation_for_returns_null_when_fallback_disabled(): void
+    {
+        $doc = Document::factory()->create(['default_locale' => 'en']);
+        $this->assertNull($doc->translationFor('ja', fallback: false));
+        $this->assertNotNull($doc->translationFor('ja', fallback: true));
+    }
+
+    public function test_available_locales_lists_existing_translations(): void
+    {
+        $doc = Document::factory()->create(['default_locale' => 'en']);
+        DocumentTranslation::factory()->create(['document_id' => $doc->id, 'locale' => 'ja']);
+        DocumentTranslation::factory()->create(['document_id' => $doc->id, 'locale' => 'fr']);
+
+        $locales = $doc->fresh()->availableLocales();
+        sort($locales);
+        $this->assertSame(['en', 'fr', 'ja'], $locales);
+    }
+}